Method and device for length measurement of packaging webs

ABSTRACT

The present invention concerns a method and a device for absolute measurement of a print repeat length and/or at least one package length of a moving packaging web ( 1 ) in line with a printing machine. Normally the print repeat length and one package length are measured and analysed simultaneously. The measurements are made by means of a number of CCD-cameras ( 2, 3, 4 ) connected to a computer ( 6 ) by means of a frame grabber ( 7 ).

FIELD OF THE INVENTION

[0001] Technical Field

[0002] The present invention concerns a method and a device for imagemeasuring of a package web in line with a printing machine.

{TECHNICAL} BACKGROUND [OF THE INVENTION

[0003] If the package length of a packaging web is not correct there maybe trouble at the filling machine. Thus, it is important to check thepackaging web concerning the package length and possibly the printrepeat length. The package length (PL) is defined as the length of eachpackage in the packaging web. The print repeat length (PRL) is definedas the length of each turn of the printing roller on the packaging web.

[0004] It is previously known to use photocells often combined with apulse counter to measure package lengths or the like on packaging web.As photocells measure only in one point the position of the photocellshas to be amended for measurement of different package webs.Furthermore, when using a pulse counter the package length is onlymeasured indirectly and the measurement may be inaccurate due to slipbetween the web and the rollers or the like.

{THE} [SUMMARY OF THE] INVENTION

[0005] One object of the present invention is to avoid the drawbacks ofthe prior art and to make absolute measurements on the package web. Afurther object is to control the PL and PRL for a packaging web in linewith a printing machine.

[0006] Still a further object is to be able to measure both PRL and onePL simultaneously and to analyse the measurements simultaneously. Yet anobject is to be able to mea sure on different packaging webs withouthaving to make any physical adjustments of the measurement units.

[0007] The above objects are achieved by a method and a device havingmeans for absolute measurement of at least the package length on apackaging web in line with a printing machine.

[0008] Further objects and advantages of the present invention will beobvious for a person skilled in the art when reading the detaileddescription below of preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The enclosed FIG. 1 is a diagrammatical view of a deviceaccording to the invention.

DETAILED DESCRIPTION OF {PREFERRED EMBODIMENTS}[THE INVENTION

[0010] In the embodiment of FIG. 1 three CCD-cameras 2,3,4 are placedabove a moving packaging web 1. The cameras 2-4 are placed in line witha printing machine (not shown) to take images of the packaging web 1.The cameras 2-4 cover the area where register codes are placed on theweb 1. Each camera 2-4 is furnished with a matrix by means of which thedifferent positions of the images are correlated to positions on the web1 and whereby image effects such as distortion are compensated for.

[0011] In other embodiments of the invention another number of camerasis used. Depending on the resolution of the camera only one camera maybe used or more than three cameras may be used covering the interestingregions.

[0012] The images of the cameras 2-4 are taken by means of a framegrabber 7. The images are then analysed by a computer

[0013] The computer 6 displays information on a monitor 9. The computer6 and frame grabber 7 are placed in a rack, e.g. a VME rack, togetherwith an I/O board 5 and a possible connection to an ethernet 8. The I/Oboard is connected with a load cell measuring the tension of the web 1,also the speed of the web 1 is measured.

[0014] To illuminate the areas covered by the cameras 2-4 a flash unit10 is provided connected to the frame grabber 7 and light units 11. Theflash unit 10 and the frame grabber 7 are coordinated to illuminate theweb 1 in time for the exposures.

[0015] In the print the web 1 is provided with register 5 codesindicating the ends of each print repeat length (PRL)and each packagelength (PL). The purpose of the system is to identify and determine theposition of the register codes. The form of the register codes will varyto be able to identify different positions on the web. The registercodes may also vary between different webs.

[0016] In the shown embodiment the first CCD-camera 2 will cover a weblength of about 180 mm. The second and third CCD-cameras 3,4 will covera partly overlapping web length of about 340 mm. The area covered by thesecond and third cameras 3,4 are placed at a distance from the centre ofthe first camera 2 which correspond to the expected PRL for a number ofdifferent packaging webs 1. The area covered by the second and thirdcameras 3,4 is big enough to cover at least one PL for each foreseenpackaging web 1. The measuring unit of FIG. 1 is envisaged to covermeasurements of different packaging webs 1, having a PL of about 110 to340 mm and a PRL of about 470 to 750 mm. On every PRL there will beabout 2 to 6 PL.

[0017] The cameras 2-4 should be mounted with the pixel 25 columnsapproximately parallel with the bars in the register code. One pixel inthe camera should correspond to a certain distance of the web 1, e.g.0.25 mm. The cameras 2-4 should be free of vibrations and for thespecific material they should have the register codes in the middle oftheir covered areas.

[0018] The light units 11 should be mounted close to the packaging web1. Furthermore, they should be mounted so they do not interfere with theregions of interest seen by the cameras 2-4 and that no direct lightfrom the light units 11 reaches the cameras 2-4. The light units shouldbe mounted in such a way, that the register codes for the specificmaterial are in the middle of the illuminated area.

[0019] In order to have accurate measurements the packaging web 1 shouldnot vibrate in the areas seen by the cameras 2-4 and should not movesideways more than ±4 mm.

[0020] The frame grabber 7 should be able to grab pictures from thethree cameras 2-4 simultaneously. The I/O-board 5 should have severaldigital inputs and outputs (trigger, frame grabber, illumination), twotimers for trigging of the frame grabber 7 and illumination(separately), at least two A/D channels (to convert the incoming webspeed signal, 0-10V) and at least two D/A converters (to be able tocontrol the illumination). The system should be able to run in at leastthe following modes:

[0021] 1. Image grabbing mode

[0022] 2. Calibration mode.

[0023] 3. Package length measurement mode.

[0024] 4. Synchronising mode.

[0025] 5. Print repeat and package length measurement mode.

[0026] 6. Automatic mode.

[0027] During the image grabbing mode exposures are made of either onecamera at the time or all three cameras simultaneously. The overlappedarea of the second and third cameras 3,4 is adjusted in such a way thatthe positions of the cameras 3,4 give a {“distortion free”}[‘distortionfree’ ] overlap. Correlation could be used during the adjustment of thecameras to optimise the overlapped area. Under the assumption that thecameras have a stable mounting, it will be enough to calculate the meanvalue of the pixel values in the overlapped region during themeasurements of PRL and PL.

[0028] During the calibration mode a calibration of the system isperformed by means of a ruler. The ruler contains several registercodes. The calibration is performed for several positions of the rulerin the fields of view of the cameras 2-4. For each position of the rulerthe positions of the register codes on the ruler are determinedaccording to the routine for accurate determination of a register codeposition (defined below). As the real distances are known, a look-uptable can be made and stored in a database. This table is then usedduring the measurements. For positions of register codes falling inbetween the calibrated positions in the look-up table, interpolation isused.

[0029] During the package length measurement mode the pictures from thesecond and third cameras 3,4 are analysed to find the two closestregister codes. For small packages there are always at least tworegister codes seen. PL should in that case be determined for theregister codes closest to the first camera 2. For big packages it mighthappen that only one code is seen. If only register code is seen, theimages are dropped and a new set of images is taken after a randomiseddelay of time. Then a new analysis is made. The above steps are repeateduntil two register codes are present. When two register codes are found,the distance between them are determined and a lock-up table (stored) isused to determine the package size. As the package size is known thedefault PRL is known as well.

[0030] During the synchronising mode the image of the first 25 camera 2is analysed to find a register code. The images of the other cameras 3,4are also stored. If no register code is found the images are dropped anda new set of images is taken after a randomised delay of time. This isrepeated until a register code is found and placed close to the centreof the image of the first camera 2. If the register code is not close tothe centre of the first camera 2, it is impossible to determine the PRLfor some package sizes. As the default PRL and the web speed are known,the time delay between the exposures are calculated and controlled insuch a way that the register code is forced to move towards the centrein the next exposure. A new measurement is normally made for each PRLand one PL. The regions of interest (ROI) analysed in the images fromeach camera 2-4 could be minimised as the approximate position of theregister codes in each picture is known. As the ROI:s are minimised thedemands of the computer for analyse are limited.

[0031] During the PRL and PL measurement mode a set of pictures istaken. The regions of interest are analysed in each picture to find anddetermine the position of the register codes and to calculate PRL andPL.

[0032] During the automatic mode the system automatically will changefrom the PL measurement mode to the synchronisation mode and to the PRLand PL measurement mode. As soon as the PL is determined thesynchronisation mode is started. As soon as the synchronisation iswithin the predetermined limits the PRL and PL measurement mode isstarted. As the approximate positions of the register codes are knownthe ROI of each image is minimised to improve the image processingspeed. If one register code is missing in any of the defined ROI a newset of pictures is taken and analysed. If there still is a code missinga restart of the PL measuring mode will be done followed by thesynchronisation mode and so on.

[0033] The time delay between exposures is set so that the web 1 “jumps”forward with one package for every new measurement. This is done to geta measurement of each PRL and PL present on the specific material. Thismay be done in the following way. First the PRL is measured by using theregister codes at the ends of each PRL. The next measurement of PRL ismade between the first register code after the end code on two adjacentprint lengths and so on until the PRL has been measured from allregister codes. During each measurement of the PRL one measurement ofthe PL is made by means of the second and third cameras 3,4. Thus, afterthe above cycle the PL of each package in one PRL is measured. Thecalculation of the time delay may be done as all the parameters areknown (PRL, PL, web speed, position of the register code in the firstcamera 2).

[0034] By the method and the device of the invention the PRL and PL:s ofdifferent packaging webs 1 are measured absolute, i.e. PRL and PL isdetermined by direct measurement on the actual web. Furthermore, severalregister codes at different locations on the web are measuredsimultaneously.

[0035] One of the digital outputs in the I/O-board is used to triggerthe exposures. The routine is different in the different modes. In thePL measurement mode it is randomised until two register codes are seenwith the second and third cameras 3,4. In the synchronising mode it israndomised until one register code is seen with the first camera 2. Inall other modes the time is calculated and thereby controlled by thesystem. The time interval within which the time should vary randomly ispreferably about 25% of the time it takes for 750 mm of the packagingweb 1 to pass at the web speed at the time for measurement. Both thetime between measurements and the time limits for the random variationis inversely proportional to the web speed. Furthermore the exposuretime may be changed manually. Preferably the exposure time should bepossible to vary between 10 μs and 200 μs. The length of the triggerpulse to the cameras 2,3,4 defines the exposure time. Normally the sametrigger is used to the cameras 2,3,4, the frame grabber 7, and theillumination 10,11. The routines are equal to all three outputs, but thetime for triggering and the pulse length could be different in all threecases. In an alternative embodiment a single trigger is used to eachoutput.

[0036] One A/D-channel of the I/O-board is implemented to read the webspeed once every measurement cycle. It is assumed that the read value islinearly proportional to the web speed. Normally 0-10 volts correspondto 0-1000 {m/mm} [m/min], but the web speed value needs to be calibratedfor each machine. It will be enough to multiply the incoming value witha factor determined by a manual calibration. This factor is manually setin the graphical user interface (GUI). The speed value is filtered by amean value calculation over a number of the latest measurements, e.g.the eight latest measurements.

[0037] The image processing part contains three different 10 routines:One for finding the correct register code size. One for determining itsposition roughly. One for determining its position accurately.

[0038] The register codes exist in a number of different sizes whosedata is stored in a database. The size of the codes depends on thepackage size of the material. At start up the size of the code isunknown. Initially the regions of interest (ROI) of the second and thirdcameras 3,4 are set to cover two stripes. These ROI will make itpossible to find the size of register codes as they cover the areaswhere the bars of the register codes will pass. The codes have three andfour bars respectively. One threshold is used to turn all pixels blackbelow that threshold and the rest white. The data is reduced by summingall columns to one row for each of the ROI. It is normally enough to useonly one ROI to determine the register code size. To find the registercode(s) correlation is used, i.e. the peak value(s) is found in acorrelation curve between the data and a known reference pattern. Inpractice a lot of multiplications and summations are done over and overagain while the reference pattern is moved, one column at the time, fromone end of the row of data to the other. This is repeated with allregister code sizes. A bipolar reference pattern is used, i.e. the blackparts are negative and the white parts are positive. There will be asignificant difference in the peak values for the different registercodes. This algorithm is based on the assumption that there are nosimilar patterns in the different print of the packages. Thereby it willbe no problem to determine which register code that is used on thematerial.

[0039] For the rest of the measurements the following method is used.First the position is determined roughly. As the reference pattern issymmetrical and placed in the middle of the correlation window (thewidth of the window varies with the size of the register code), thepeak(s) of the correlation curve determines the position of the registercode(s). The middle of the register code is actually placed a halfwindow size to the left or to the right of the peak, depending on fromwhich direction the correlation is performed. Each peak, above anempirically determined threshold, tells the position of each registercode if more than one is detected in the area. Thereafter, PL isdetermined roughly by calculating the number of columns between twopeaks and multiplying with the distance each pixel corresponds to,nominally 0.25 mm. Finally, the position is determined accurately. Thismay be done in different ways depending on in which mode the system isrunning. In the PL mode and the synchronisation mode it is enough with arough determination of the register code positions. In the combined PRLand PL mode the register codes are accurately determined by using fivethresholds in finding each edge of each bar in the register code. Inthis mode the ROI:s are different from the ROI:s of the previous modes,as the positions of the register codes now are roughly known. Nocompressing of data to one row is used, as the codes not necessarily areparallel with the columns in the CCD cameras 2,3,4.

[0040] In a data base data for the different register codes sizes, thePL for different packages and the PRL for different packages are stored.

[0041] The graphical user interface (GUI) is made with the purpose tomake it easy to perform all above defined functions, measurements andparameter settings. It is also made in a way that makes it easy toverify the functions and the performance of the system. Results arenormally presented in histograms and diagrams and are stored on file forboth PRL and PL measurements.

1. (Amended) A method for measurement on a moving packaging web, whereinthe absolute value of a print repeat length (PRL) and/or at least onepackage length (PL) are measured on the web in line with a printingmachine.
 2. (Amended) The method of claim 1, wherein the measurementsare made by means of printed register codes on the web, which registercodes are printed at the end of each package length and print repeatlength and/or that the print repeat length and at least one packagelength are measured and analysed simultaneously.
 3. (Amended) The methodof claim 1, wherein one or more CCD-cameras is used to take images ofregions of interest on the web, which regions are regions including oneor more register codes.
 4. (Amended) The method of claim 3, wherein thetime intervals at which the images are taken are synchronised in view ofthe actual data concerning package size, print repeat length, web speed,web tension etc in such a way that a register code for print repeatlength is centred in the image of the first camera.
 5. (Amended) Themethod of claim 1, wherein second and third CCD-cameras are placedadjacent each other at a distance from a first CCD camera; that thesecond and third cameras cover a partly overlapped region; and that thedistance between the images taken by the first camera and the second andthird cameras covers the print repeat length for a 30 number ofdifferent packaging webs.
 6. (Amended) The method of claim 5, whereinthe region covered by the second and third cameras is big enough toenclose at least one print length; that the overlapped area of thesecond and, third cameras is calibrated; and that each print repeatlength of the web is measured.
 7. (Amended) The method of claim 1,wherein the region of interest which is analysed on each image isminimised by means of stored sizes for different package sizes and byanalysing the actual web speed and tension and the latest precedingmeasurements and that the regions of interest are illuminated duringimage taking.
 8. (Amended) The method of claim 7, wherein illuminationis made by means of a flash unit trigged automatically.
 9. (Amended) Themethod of claim 1, wherein the images are transferred to a computer bymeans of a frame grabber and that all cameras take imagessimultaneously, which images are analysed simultaneously.
 10. (Amended)The method of claim 1, wherein at least the following modes areavailable: an image grabbing mode, a calibration mode, a package lengthmeasurement mode, a synchronising mode, a print repeat and packagelength mode and an automatic mode, and that in the latter mode thesystem automatically changes between the package length measurementmode, the synchronising mode and the print repeat and package lengthmode.
 11. (Amended) The method of claim 10, wherein during thesynchronisation mode the timing of the exposures of each camera isadapted to the actual measured lengths and condition of the web and thatstored values regarding register codes, package lengths and print repeatlengths for different packages are used to control the timing of themeasurements.
 12. (Amended) A length measuring device for a movingpackaging web, wherein means are provided for absolute measurement of aprint repeat length (PRL) and/or at least one package length (PL) of thepackaging web in line with a printing machine.
 13. (Amended) The deviceof claim 12, wherein means are provided for simultaneous measurement ofthe print repeat length and at least one package length and/or that themeans for measurements are one or more CCD-cameras connected to acomputer by means of a frame grabber.
 14. (Amended) The device of claim13, wherein a first CCD-camera is positioned at a distance from at leasttwo CCD-cameras, which distance corresponds to the print repeat lengthof a number of different packaging webs.
 15. (Amended) The device ofclaim 14, wherein the second and third CCD-cameras together covers apartly overlapping area, which area is big enough to include at leastone package length and that means are provided for measurement of thespeed and tension of the package web.
 16. (Amended) The device of claim12, wherein means are provided for illumination of the areas in whichimages are taken by the cameras.
 17. (Amended) The device of claim 16,wherein the illumination means are light units controlled by thecomputer by means of a flash unit for co-ordination with the cameras.